The present invention relates to an optical filter comprising an integrated wavelength dispersive element having an input for providing temperature compensation, particularly for providing passive temperature compensation in an arrayed waveguide grating.
An arrayed waveguide grating (AWG) is a dispersive optical device used for multiplexing or demultiplexing a set of optical telecommunications channels having different wavelengths. An example of an AWG is shown in FIG. 1. The AWG 100 is an integrated optics device formed on a substrate. The AWG has at least one input waveguide 10 for launching a multiplexed signal comprising a plurality of channels at specific wavelength bands having center wavelengths xcex1 to xcexn, into a free-space slab, or planar waveguide, such as a star coupler 12. The star coupler 12 distributes a wavefront of the signal evenly to a plurality of waveguides that form the grating 14. Each of the plurality of waveguides has a different optical length, the optical lengths of adjacent waveguides differing by a constant value and increasing geometrically from one side of the grating to the other. Interference caused by the relative phase differences introduced by the grating 14 occurs in a second free-space slab, or planar waveguide, such as a star coupler 16. The dispersion of the grating 14 physically separates the different wavelengths and focuses the dispersed light on an output plane 17 of the second star coupler, where separated wavelengths are coupled into a plurality of output waveguides 18. A center wavelength of a selected channel is located at a selected output waveguide 18 for optimized coupling. The center wavelength and the spacing of the individual wavelength bands of the channels are determined by the geometry of the AWG layout and by the effective refractive index of the waveguides of the grating. The output waveguides 18 determine the bandwidth of the individual channels by their width. Focus points T1O and T2O at the output plane 17 of the output planar waveguide 16 demonstrate the wavelength shift of the center wavelength that occurs as a result of a change in temperature of the device 100 with the input point fixed.
Operated in a reverse direction, multiple signals of different wavelengths are launched from the plurality of waveguides 18 and pass through the grating 14 to interfere in the star coupler 12, and be combined as a multiplexed signal into a single waveguide 10.
The position of the input waveguide 10 at the input plane 20 of the star coupler 12, from which a multiplexed signal is launched, affects the location of the focused output signals. Input waveguides have been included as a part of the integrated device. However, manufacturing tolerances are not tight enough to accurately set the center wavelength in manufacture for narrow channel spacing. The index accuracy achieved with the many deposition techniques used to make AWGs is not sufficient to set the central wavelength within the required tolerances.
In U.S. Pat. No. 5,732,171, assigned to Siemens Aktiengesellschaft, Michel et al. disclose placing the input plane of the star coupler at the edge of the substrate in which the device is formed to permit coupling a waveguide at a selected location after manufacture. Tuning may be performed to align the center wavelength of the channels of the multiplexed signal with their respective output ports to optimize coupling.
Tuning by affixing a fiber pigtail is subject to alignment error over 5 degrees of freedom. With reference to FIG. 1, X-Y-Z coordinates are shown. The X axis indicates lateral movement along the input plane 13 of the star coupler 12, which affects the center wavelength alignment. The Y axis indicates vertical movement with the planar slab, which is generally single mode in the vertical direction. Consequently fine alignment is necessary to reduce coupling losses. The Z axis indicates movement in and out from the input plane 20 of the star coupler 12. Alignment in this axis affects the pitch, or separation of the focused channel outputs on the output plane 17 of the second star coupler 16. In addition xcex8X and xcex8Y indicate rotational tilt about the X and Y axes, which will further affect tuning of the center wavelength and insertion loss.
A further problem in tuning the AWG is the temperature dependency of the device. Temperature change causes the refractive index of the phased array to change. This causes the wavelength bands of the channel outputs to shift position. Consequently, coupling to the output waveguides is not efficient at the center wavelength.
One solution to this problem is proposed by the present inventor in U.S. Pat. No. 5,905,824, which teaches providing an arrayed waveguide grating and a separate output waveguide chip optically coupled to the output planar waveguide of the AWG, with passive thermally responsive means for relative movement between them, or through an imaging lens passively positioned between them. Although this device provides passive temperature compensation, it does not provide means for adjusting the input waveguide for tuning the center wavelength.
Passive temperature compensation at the input of an AWG is proposed in a paper entitled, xe2x80x9cOptical Phased Array Filter Module with Passively Compensated Temperature Dependence,xe2x80x9d by G. Heise et al. of Siemens AG, presented at ECOC ""98, 20-24, Sep. 1998 in Madrid, Spain. Heise et al. propose supporting a fiber lens pigtail adjacent the input plane of the planar waveguide using a thermal expansion rod secured to the substrate of the AWG. The thermal expansion rod provides lateral displacement of the input fiber pigtail. However, as discussed with respect to the earlier Siemens patent, alignment of the fiber pigtail is subject to alignment and coupling error over five degrees of freedom. In order to permit lateral movement of the input pigtail, a gap between the input plane and the fiber is required. Without securing the fiber to the substrate, the likelihood of misalignment is increased. In addition, the air gap between the fiber and the input slab will increase insertion losses and introduce additional problems of back reflection.
It is desired to provide an improved coupling into an arrayed waveguide grating, which will permit variable tuning to adjust the center wavelength and provide passive temperature compensation.
It is further desired to provide an arrayed waveguide having an integrated variable input waveguides to provide tuning flexibility.
The present invention has found that by providing an integrated wavelength dispersive element having a thermally responsive pivotal input structure for changing an angle of a collimated input signal launched into a focusing lens, the input point can be selected in response to changing temperature in order to compensate for thermal drift of the center wavelength. Further, the present invention has found that by providing a reflective lens assembly for focusing an input signal at a selected input point of the input planar waveguide, alignment and tuning of an input and assembly can be improved and simplified.
Accordingly, the present invention provides an input coupling for launching light into a planar waveguide of an integrated wavelength dispersive element comprising:
focusing means having optical power for focusing light at an input point on the input plane of the planar waveguide;
an input waveguide for launching a signal comprising a plurality of channels at specific wavelengths into the integrated wavelength dispersive element;
means for coupling the signal as a beam into the focusing means; and,
tilt means including a pivotal structure having a center of rotation and a thermally responsive actuator, for imparting a tilt on the beam coupled to the focusing means in response to a change in temperature.
In an alternative embodiment the present invention provides an arrayed waveguide grating comprising:
a substrate for supporting an integrated arrayed waveguide grating formed therein including:
an input planar waveguide, having an input plane at an edge of the substrate and an output plane, for propagating a wavefront from an input point on the input plane to an output plane;
a grating comprising an array of waveguides optically coupled to the output plane of the input planar waveguide for receiving the wavefront, an optical length of the waveguides differing by a substantially equal amount from a first waveguide to an nth waveguide; and,
an output planar waveguide for focusing separated wavelength signals on an output plane of the output planar waveguide for coupling to output waveguides; and
an input coupling for launching a signal into the integrated arrayed waveguide grating including:
at least one input waveguide;
a lens for focusing an input signal at the input point of the input planar waveguide.
means for coupling the signal as a collimated beam into the lens; and,
tilt means including a pivotal structure having a center of rotation and a thermally responsive actuator, for imparting a tilt on the collimated beam at a focal plane of the lens in response to a change in temperature.
Advantageously, a passive thermally responsive input coupling is provided which also facilitates initial center wavelength tuning of the device.
As an additional advantage, variable coupling parameters can be incorporated into a reflective coupling including input position, waveguide taper and planar waveguide length increment to provide relatively simple tuning in an integrated device.
While an arrayed waveguide grating is a more commonly used integrated wavelength dispersive element used in the telecommunications industry, an echelle grating is also an integrated wavelength dispersive element equally adapted for use with the input coupling in accordance with the present invention.